In a variety of telecommunications and other applications, batteries, e.g., valve-regulated lead acid (VRLA) batteries, are employed to provide reserve energy to the equipment powered thereby. With the increasing trend toward distributed power systems, battery reserve systems are often remotely located in outdoor uncontrolled environments. When deployed in outdoor environments, the batteries are generally placed in closed cabinets with poor heat-exchange characteristics. The batteries are, therefore, exposed to high temperatures with poor ventilation.
While reducing the temperature of the operating environment of the battery is an important factor in sustaining the life of the battery, there are other ancillary considerations as well. The system employed to maintain the battery in a state of readiness, i.e., fully charged, is another important consideration in battery reserve systems. Generally, the battery is "floated" or connected to a rectifier that also provides power to the electrical load and the output voltage of the rectifier is selected to maintain the battery in a fully charged condition. If the DC power from the rectifier is interrupted, the battery will immediately provide power to the load. The uninterruptable supply of power is typically an important consideration in the design of battery reserve systems. Since elements of the battery experience aging during float charging, e.g., excess current contributes to grid corrosion of the positive plate of the battery and water loss, it is advantageous to decrease the period of time that the battery is in the float mode.
In "A New Concept: Intermittent Charging of Lead Acid Batteries in Telecommunication Systems," by D. P. Reid, et al. (Reid), Proceedings of INTELEC 1984, pp. 67-71, which is incorporated herein by reference, an intermittent charging system is disclosed. Since the commercial AC power source is typically available about 99.9% of the time, the battery is conventionally maintained in a float mode whereby the battery is fully charged and is essentially being topped-off continuously. With an intermittent charging system, the battery is only charged a fraction of the time and, otherwise, the battery is disconnected from the charging circuit. In other words, the life of batteries may be extended by reducing the time the batteries are on float. This is accomplished by maintaining the batteries at their open circuit potential for most of the time with periodic recharging. As disclosed in Reid, the life of a battery may double by employing a 50% float duty cycle over a full float duty cycle operation for a particular battery design. Therefore, a reduction in the float mode duty cycle significantly increases the life of the battery.
Additionally, it has been found highly undesirable to allow the batteries in the battery strings to discharge completely. When a battery of the type employed in such units is completely discharged, it becomes impractical to recharge, and the batteries are usually discarded. This is both expensive and wasteful. It is far better to discharge a battery string only as far as possible without permanently damaging it and then to disconnect the battery string to prevent it from being discharged further. Accordingly, commercially-available battery backed-up power units frequently provide a low voltage disconnect (LVD) that detects the output voltage of each battery string or group of parallel strings and, in response thereto, decouples battery strings as their voltage drops below a predetermined minimum level.
Accordingly, what is needed in the art is a solution to the above-described battery life problem. More specifically, what is needed in the art is a line-replaceable battery disconnect circuit that can provide such solution.